Device for projecting ink droplets onto a medium

Abstract

A device for projecting ink droplets through a set of projection holes to print a pattern under selective electrical or light pulse control. A first plate formed with a large number of holes is placed in close proximity to the surface of the printing medium. A second plate a small distance from the first defines therewith a chamber in which the ink is locally heated by electrical current controlled by light selectively impinging on a photoconductive part of said second plate in register with each hole or each group of holes so as to form a pattern with constant or variable parts.

I claim: 
     
       1. A device for projecting ink droplets onto a medium so as to form dots defining one or more patterns, said dots being selected from an array of dots, the device comprising a perforated first plate which in use is adapted to be positioned substantially parallel to and spaced by a small distance from the medium, a second plate attached and substantially parallel to the first plate and defining therewith a chamber for containing ink to be deposited on the medium, and means for projecting from a selected hole in said perforated first plate a small quantity of ink by the application of voltage to at least one plate in conjunction with the passage of electromagnetic radiation through the second plate, wherein the improvement comprises: said second plate comprises a layer of photoconductive material whose electrical resistivity is reduced by exposure to electromagnetic radiation and a layer of electrically conductive material which is transparent to said radiation disposed in current conductive relation to the side of said photoconductive layer opposite the ink chamber side and   an electrically conductive member disposed on the ink chamber side of said photoconductive layer, said device being adapted to have a preselected voltage applied between said electrically conductive layer and member, the value of said voltage being chosen such that exposure of a region of the photoconductive layer registered with a selected hole in said perforated first plate to electromagnetic radiation of predetermined intensity will permit sufficient current flow between the electrically conductive layer and member through said region of photoconductive material to expel an ink droplet from the selected hole as a result of local thermal expansion of the ink.   
     
     
       2. A device according to claim 1, wherein said electrically conductive member comprises a second layer of electrically conductive material disposed in current conducting relation to the ink chamber side of said layer of photoconductive material. 
     
     
       3. A device according to claim 1 or 2, further comprising a set of projections on at least one of said first plate and said second plate partially filling the space between said plates, the height of said projections being equal to or less than the distance separating the first and second plates and the projections being regularly spaced between the axes of holes in said first plate. 
     
     
       4. A device according to claim 1 or claim 2 wherein said first mentioned electrically conductive layer has an electrical resistivity of between 10 -6  ohm-meters and 50 ohm-meters. 
     
     
       5. A device according to claim 1, wherein said photoconductive material is selected from the group consisting of silicon, germanium and cadmium sulphide. 
     
     
       6. A device according to claim 1, further comprising a set of electrically-controlled liquid crystal cells located between the source of said radiation and said photoconductive material, and means for selectively controlling the liquid crystal cells to permit the radiation to impinge on selected parts of the photoconductive layer. 
     
     
       7. A device according to claim 1, further comprising a mask with sections transparent to said radiation and sections opaque thereto located between a source of radiation and said photoconductive material. 
     
     
       8. A device according to claim 1, further comprising a supply of ink filling said chamber, wherein the ink is solid at the normal temperature of operation and is melted by the local increase in temperature resulting from passage of current between said electrically conductive layer and member through a radiation-impinged region of the photoconductive material. 
     
     
       9. A device according to claim 1, wherein said electrically conductive member comprises at least part of the first plate. 
     
     
       10. A device according to claim 9, further comprises a supply of electrically conductive ink filling said ink chamber. 
     
     
       11. A device according to claim 10, wherein the electrical resistivity of the ink is between 50 and 0.05 ohm-meters.